Today's communications systems utilize various types of cable and radio interfaces. The most reliable are glass optical fibres which also enable very high transmission rates. On the other hands, copper cables still form part of the telephone lines which are also used for transmission of data. Especially in the last decades, wireless communications developed rapidly. All these data transport media have their own characteristics and are suitable for deployment in different scenarios and architectures.
Glass optical fibres (GOF) are used nowadays especially for communication requiring a very high bandwidth and very low attenuation. Since glass optical fibres have very small diameters and low numerical apertures (NA) its installation requires special and expensive connector tools and skilled installation workers.
Another possibility is the deployment of plastic optical fibres (POF), for instance, based on polymethacrylate (PMMA) with a larger core diameter (about 1 mm) and a high numerical aperture (NA of approximately 0.3 to 0.5). The least expensive and most used plastic optical fibre is an SI-POF with a numerical aperture of 0.5. However, there is also an SI-POF with a low numerical aperture of 0.3 enabling higher data rates as well as PMMA GI-POF with a bandwidth length product near to 1 GHz×100 meter. PMMA has several attenuation windows that enable POF to be used with different visible light sources from blue to red Light Emitting Diodes (LED) or red Lasers Diodes (LD).
In comparison with GOF, plastic optical fibres have an advantage of a very easy installation. They can be deployed by professional or non-professional installation workers using basic tools such as scissors or cutters and inexpensive plastic connectors. It is resilient to misalignment and strong vibrations so it can be installed in industrial and automotive environments without loss of communication capacity. The POF connections have also much higher tolerance to residual dust on the terminal faces than GOF, due to the larger core diameter.
Since the transmission over POF is optic, plastic optical fibres are completely immune to electrical noise. Thus, the existing copper wiring will not interfere with data passing through plastic optical fibres so it can even be installed next to electrical cabling. Plastic optical fibre connectors and optoelectronics for POF are mainly low cost consumer parts which enable installation workers to save cable costs and installation, testing, and maintenance time. Plastic optical fibres have been widely employed, in particular, for infotainment networks in cars and can now be seen as a global standard for high-speed on-board car networks such as Media Oriented Systems Transport (MOST).
FIG. 1 illustrates an example of a system for transmission and reception of data over POF. The transmission over plastic optical fibres is based on a light intensity modulation with direct detection. The signal to be transmitted is generated from a digital circuit 110 for encoding and modulating the user bit stream information and passed to a transmitter (Tx) analogue front end (AFE) 120 for conversion of digital data into an electrical signal for controlling the light emitting element 130. After this conversion of the electric signal to an optical signal, the latter is then input to the optical fibre 150. Electrical optical converters used for plastic optical fibres are typically light-emitting diodes (LED) characterized by properties such as a peak wavelength, a wavelength width or launching modal distribution.
During the transmission of the signal via plastic optical fibres 150, the light is affected by severe attenuation as well as distortion mainly due to modal dispersion. The modal dispersion is caused by different modes of light propagating in the fibre on different paths and with different speeds and attenuations, resulting in different arrival times at the receiver. The optical signal is also affected by a so-called mode coupling where the energy of higher order modes is transferred to lower order modes and vice versa. As a consequence, an optical pulse is broadened which leads to lower the signal bandwidth.
At a receiver, the optical signal from the plastic optical fibre 150 is converted into electrical intensity by means of an opto-electric converter 170 such as a photodiode. Then, the electrical signal is processed by the analogue front end (AFE) 180. In particular, it is amplified, inter alia by a trans-impedance amplifier (TIA) and connected to a digital receiver 190. The TIA is typically the most important noise source which limits the final sensitivity of the communication system.
Regarding the data transmission technology, GOF have been successfully using a non-return-to-zero (NRZ) modulation. In particular, current glass fibre communication systems mainly utilize NRZ 8b/10b or NRZI 4b/5b line coding which requires a baud rate of 1.25 GHz and 125 MHz for 1 Gbps and 100 Mbps solutions, respectively. Current plastic optical fibre solutions thus also adopted NRZ modulation for data communications. However, plastic optical fibres have a frequency and time response different from that of glass fibres and also have considerably higher attenuation. As a communication medium, plastic optical fibres show a very high modal dispersion due to its important differential mode delay and differential mode attenuation. The large area photodiodes required for coupling with a fibre typically have a limited bandwidth. In view of a plastic optical fibre frequency response, solutions supporting 100 or 150 Mbps are possible up to ca. 50 meters; however, 1 Gbps does not seem to be achievable without a more advanced technology.
FIG. 2A shows a variation of POF optical bandwidth (y axis, in MHz) as a function of the fibre length (x axis, in meters). FIG. 2B shows the variation of the bandwidth-length product (y axis, in MHz·100 m) as a function of the fibre length. Here, the fibre is an SI-POF with a numerical aperture NA of 0.5 (in particular, model Mitsubishi Eska-GH4001), and the light source is an RCLED with launching condition FWHN NA of 0.31, wavelength peak of 658 nanometers and an FWHN wavelength width of 21 nanometers. As can be seen from FIG. 1, a suitable flat response for a desired 1.25 GHz baud rate is only possible in the very first meters of the plastic optical fibre. For a laser light source, the optical bandwidth as a function of length is very similar. Therefore, the bandwidth bottleneck is produced by plastic optical fibres independently on how fast the light source is because the limiting factor is, in particular, the modal dispersion by mode coupling in the fibre.